U.S. patent application number 16/718402 was filed with the patent office on 2020-06-25 for fuel cell system.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Masayuki ITO, Tomohiko KANEKO.
Application Number | 20200203739 16/718402 |
Document ID | / |
Family ID | 68848106 |
Filed Date | 2020-06-25 |
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United States Patent
Application |
20200203739 |
Kind Code |
A1 |
KANEKO; Tomohiko ; et
al. |
June 25, 2020 |
FUEL CELL SYSTEM
Abstract
A fuel cell system includes: a fuel cell; a supply device; and a
control unit configured to lower output voltage of the fuel cell to
a target value so as to execute recovery processing to recover
power generation performance of the fuel cell. In a case of having
an execution request of the recovery processing, the control unit
restarts power generation of the fuel cell when open circuit
voltage of the fuel cell is lowered to or smaller than a threshold
value higher than the target value by controlling a flow rate of a
cathode gas while the power generation of the fuel cell is paused,
and the control unit executes the recovery processing while
controlling an output current value of the fuel cell to be smaller
than an idle current value that is an output current value of the
fuel cell in an idle operation state.
Inventors: |
KANEKO; Tomohiko;
(Okazaki-shi, JP) ; ITO; Masayuki; (Sunto-gun,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
68848106 |
Appl. No.: |
16/718402 |
Filed: |
December 18, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2250/20 20130101;
H01M 8/04582 20130101; H01M 8/04776 20130101; H01M 8/04552
20130101; H01M 8/04753 20130101; H01M 8/04791 20130101; H01M 8/0488
20130101; H01M 8/04395 20130101 |
International
Class: |
H01M 8/0438 20060101
H01M008/0438; H01M 8/04746 20060101 H01M008/04746; H01M 8/04537
20060101 H01M008/04537 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2018 |
JP |
2018-241433 |
Claims
1. A fuel cell system comprising: a fuel cell; a supply device
configured to supply a cathode gas to the fuel cell and adjust a
flow rate of the cathode gas supplied to the fuel cell; and a
control unit configured to lower output voltage of the fuel cell to
a target value so as to execute recovery processing to recover
power generation performance of the fuel cell, wherein: the control
unit is configured to, in a case of having an execution request of
the recovery processing, i) restart power generation of the fuel
cell when open circuit voltage of the fuel cell is lowered to or
smaller than a threshold value higher than the target value by
controlling the flow rate of the cathode gas while the power
generation of the fuel cell is paused, and ii) execute the recovery
processing while controlling an output current value of the fuel
cell to be smaller than an idle current value that is an output
current value of the fuel cell in an idle operation state.
2. The fuel cell system according to claim 1, wherein an output
current value of the fuel cell is smaller than the idle current
value, the output current value corresponding to the target value
and determined by current-voltage characteristics of the fuel cell
corresponding to the threshold value.
3. The fuel cell system according to claim 1, wherein the control
unit is configured to execute the recovery processing by restarting
power generation of the fuel cell so as to increase the output
current value of the fuel cell within a range in which the output
current value is smaller than the idle current value, and then
decreasing the output current value.
4. The fuel cell system according to claim 1, wherein the control
unit is configured to execute the recovery processing while
limiting the flow rate of the cathode gas to or less than a flow
rate of the cathode gas at which the open circuit voltage becomes
lowered.
5. The fuel cell system according to claim 1, wherein the control
unit is configured to execute the recovery processing while setting
the flow rate of the cathode gas to zero.
6. The fuel cell system according to claim 1, wherein: the control
unit is configured to, when a request output to the fuel cell is
equal to or smaller than a predetermined value, execute an
intermittent operation for pausing the power generation of the fuel
cell and controlling increase and decrease of the flow rate of the
cathode gas so as to converge the open circuit voltage within a
target range; the threshold value is within the target range; and
the control unit is configured to, during the intermittent
operation in a case of having the execution request of the recovery
processing, stop the intermittent operation and execute the
recovery processing when the open circuit voltage is lowered to or
smaller than the threshold value.
7. The fuel cell system according to claim 1, wherein: the control
unit is configured to, when a request output to the fuel cell is
equal to or smaller than a predetermined value, execute an
intermittent operation for pausing the power generation of the fuel
cell and controlling increase and decrease of the flow rate of the
cathode gas so as to converge the open circuit voltage within a
target range; the threshold value is smaller than a lower limit
value of the target range; and during the intermittent operation in
a case of having the execution request of the recovery processing,
the control unit does not increase the flow rate of the cathode gas
so as to increase the open circuit voltage even when the open
circuit voltage is lowered to the lower limit value, and stops the
intermittent operation and executes the recovery processing when
the open circuit voltage is lowered to or smaller than the
threshold value.
8. The fuel cell system according to claim 1, wherein the fuel cell
system is mounted in a vehicle; the supply device includes a
compressor configured to supply the cathode gas to the fuel cell,
and a switching mechanism configured to switch a state from a
supply state to a bypass state, the cathode gas being supplied from
the compressor to the fuel cell in the supply state, the fuel cell
being bypassed in the bypass state such that the cathode gas is not
supplied to the fuel cell and discharged to an outside of the fuel
cell; and the control unit is configured to, when a regenerative
power amount of a power generator mounted in the vehicle is equal
to or greater than a predetermined value, execute a regenerative
operation for switching the state from the supply state to the
bypassing state and pausing the power generation of the fuel cell
while consuming the regenerative power of the power generator by
driving of the compressor, and the control unit is configured to,
during the regenerative operation in a case of having the execution
request of the recovery processing, execute the recovery processing
while continuing the regenerative operation when the open circuit
voltage is lowered to or smaller than the threshold value.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2018-241433 filed on Dec. 25, 2018 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND
1. Technical Field
[0002] The present disclosure relates to a fuel cell system.
2. Description of Related Art
[0003] In some cases, recovery processing for recovering power
generation performance of a fuel cell is performed by lowering
voltage of the fuel cell to a predetermined target value (for
example, see Japanese Patent Application Publication No.
2008-130402).
SUMMARY
[0004] In the process in which the voltage of the fuel cell is
lowered to the target value as described above, since power
generation and heat generation are both continued in the fuel cell,
the power generation amount and the heat generation amount are
increased. Consequently, surplus electric power is increased and
the fuel cell may have a high temperature.
[0005] The present disclosure provides a fuel cell system capable
of executing recovery processing for recovering power generation
performance of a fuel cell while decreasing the power generation
amount and the heat generation amount.
[0006] According to one aspect of the present disclosure, provided
is a fuel cell system. This fuel cell system includes: a fuel cell;
a supply device configured to supply a cathode gas to the fuel cell
and adjust a flow rate of the cathode gas supplied to the fuel
cell; and a control unit configured to lower output voltage of the
fuel cell to a target value so as to execute recovery processing to
recover power generation performance of the fuel cell. The control
unit is configured to, in a case of having an execution request of
the recovery processing, i) restart power generation of the fuel
cell when open circuit voltage of the fuel cell is lowered to or
smaller than a threshold value higher than the target value by
controlling the flow rate of the cathode gas while the power
generation of the fuel cell is paused, and ii) execute the recovery
processing while controlling an output current value of the fuel
cell to be smaller than an idle current value that is an output
current value of the fuel cell in an idle operation state.
[0007] According to the fuel cell system in the above aspect, since
the power generation is restarted to execute the recovery
processing when the open circuit voltage is lowered to or smaller
than the threshold value, the output voltage can be lowered to the
target value with a smaller power generation amount and a smaller
heat generation amount. Furthermore, since the recovery processing
is executed by controlling the output current value of the fuel
cell to be smaller than the idle current value, the output voltage
can be lowered to the target value with a smaller power generation
amount and a smaller heat generation amount.
[0008] In the fuel cell system in the above aspect, an output
current value of the fuel cell may be smaller than the idle current
value, the output current value corresponding to the target value
and determined by current-voltage characteristics of the fuel cell
corresponding to the threshold value.
[0009] In the fuel cell system in the above aspect, the control
unit may be configured to execute the recovery processing by
restarting power generation of the fuel cell so as to increase the
output current value of the fuel cell within a range in which the
output current value is smaller than the idle current value, and
then decreasing the output current value.
[0010] In the fuel cell system in the above aspect, the control
unit may be configured to execute the recovery processing while
limiting the flow rate of the cathode gas to or less than a flow
rate of the cathode gas at which the open circuit voltage becomes
lowered.
[0011] In the fuel cell system in the above aspect, the control
unit may be configured to execute the recovery processing while
setting the flow rate of the cathode gas to zero.
[0012] In the fuel cell system in the above aspect, the control
unit may be configured to, when a request output to the fuel cell
is equal to or smaller than a predetermined value, execute an
intermittent operation for pausing the power generation of the fuel
cell and controlling increase and decrease of the flow rate of the
cathode gas so as to converge the open circuit voltage within a
target range; the threshold value is within the target range; and
the control unit may be configured to, during the intermittent
operation in a case of having the execution request of the recovery
processing, stop the intermittent operation and execute the
recovery processing when the open circuit voltage is lowered to or
smaller than the threshold value.
[0013] In the fuel cell system in the above aspect, the control
unit may be configured to, when a request output to the fuel cell
is equal to or smaller than a predetermined value, execute an
intermittent operation for pausing the power generation of the fuel
cell and controlling increase and decrease of the flow rate of the
cathode gas so as to converge the open circuit voltage within a
target range; the threshold value is smaller than a lower limit
value of the target range; and during the intermittent operation in
a case of having the execution request of the recovery processing,
the control unit may not increase the flow rate of the cathode gas
so as to increase the open circuit voltage even when the open
circuit voltage is lowered to the lower limit value, and may stop
the intermittent operation and execute the recovery processing when
the open circuit voltage is lowered to or smaller than the
threshold value.
[0014] In the fuel cell system in the above aspect, the fuel cell
system may be mounted in a vehicle, and the supply device may
include: a compressor configured to supply the cathode gas to the
fuel cell; and a switching mechanism configured to switch a state
from a supply state to a bypass state, the cathode gas being
supplied from the compressor to the fuel cell in the supply state,
the fuel cell being bypassed in the bypass state such that the
cathode gas is not supplied to the fuel cell and discharged to an
outside of the fuel cell. The control unit may be configured to,
when a regenerative power amount of a power generator mounted in
the vehicle is equal to or greater than a predetermined value,
execute a regenerative operation for switching the state from the
supply state to the bypassing state and pausing the power
generation of the fuel cell while consuming the regenerative power
of the power generator by driving of the compressor. The control
unit may be configured to, during the regenerative operation in a
case of having the execution request of the recovery processing,
execute the recovery processing while continuing the regenerative
operation when the open circuit voltage is lowered to or smaller
than the threshold value.
[0015] According to the fuel cell system in the above aspect, it is
possible to provide a fuel cell system capable of executing the
recovery processing that recovers the power generation performance
of the fuel cell while decreasing the power generation amount and
the heat generation amount.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Features, advantages, and technical and industrial
significance of exemplary embodiments of the disclosure will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements; and wherein:
[0017] FIG. 1 is a configuration diagram of a fuel cell system
mounted in a vehicle;
[0018] FIG. 2 is a graph showing an IV curve of a fuel cell;
[0019] FIG. 3A is an explanatory diagram of recovery processing of
a comparative example;
[0020] FIG. 3B is an explanatory diagram of recovery processing of
a comparative example;
[0021] FIG. 3C is an explanatory diagram of recovery processing of
a comparative example;
[0022] FIG. 3D is an explanatory diagram of recovery processing of
a comparative example;
[0023] FIG. 4A is an explanatory diagram of recovery processing of
the present embodiment;
[0024] FIG. 4B is an explanatory diagram of the recovery processing
of the present embodiment;
[0025] FIG. 5 is a flowchart illustrating an example of control of
the present embodiment;
[0026] FIG. 6 is a timing chart when the recovery processing is
executed during an intermittent operation;
[0027] FIG. 7 is a timing chart when the recovery processing is
executed during a regenerative operation;
[0028] FIG. 8A is an explanatory diagram of recovery processing
according to a first modification;
[0029] FIG. 8B is an explanatory view of the recovery processing of
the first modification;
[0030] FIG. 9 shows a timing chart when the recovery processing of
the first modification is executed during the intermittent
operation;
[0031] FIG. 10A is an explanatory view of recovery processing of a
second modification;
[0032] FIG. 10B is an explanatory view of the recovery processing
of the second modification;
[0033] FIG. 10C is an explanatory view of the recovery processing
of the second modification;
[0034] FIG. 10D is an explanatory view of the recovery processing
of the second modification; and
[0035] FIG. 11 is a timing chart when the recovery processing of
the second modification is executed during the intermittent
operation.
DETAILED DESCRIPTION OF EMBODIMENTS
[0036] Configuration of Fuel Cell System
[0037] FIG. 1 is a configuration diagram of a fuel cell system 1
mounted in a vehicle. The fuel cell system 1 includes: an ECU
(electronic control unit) 3; a fuel cell (hereinafter, referred to
as an FC) 4; a rechargeable battery (hereinafter, referred to as a
BAT) 8; a cathode gas supply system 10; an anode gas supply system
20; a power control system 30; and a cooling system 40. The fuel
cell system 1 includes a cooling system (not shown) that cools the
FC 4 by circulating cooling water. Further, the vehicle includes a
motor 50 for traveling, wheels 5, and an accelerator opening sensor
6. The FC 4 is a fuel cell generating electricity by receiving
supply of a cathode gas and an anode gas, and is configured by
stacking a plurality of solid polymer electrolyte type single
cells.
[0038] The cathode gas supply system 10 (supply device) supplies
air containing oxygen as the cathode gas to the FC4, and includes:
a supply pipe 11; a discharge pipe 12; a bypass pipe 13; an air
compressor 14 (compressor); a bypass valve 15; an intercooler 16;
and a back pressure valve 17. The supply pipe 11 is connected to a
cathode inlet manifold of the FC 4. The discharge pipe 12 is
connected to a cathode outlet manifold of the FC 4. The bypass pipe
13 communicates the supply pipe 11 and the discharge pipe 12 with
each other. The bypass valve 15 is provided at a connection part
between the supply pipe 11 and the bypass pipe 13. The bypass valve
15 switches a communication state between the supply pipe 11 and
the bypass pipe 13. The air compressor 14, the bypass valve 15, and
the intercooler 16 are arranged on the supply pipe 11 in this order
from the upstream side. The back pressure valve 17 is disposed on
the discharge pipe 12 and located more upstream than the connection
part between the discharge pipe 12 and the bypass pipe 13. The air
compressor 14 supplies air containing oxygen as the cathode gas to
the FC 4 via the supply pipe 11. The cathode gas supplied to the FC
4 is discharged through the discharge pipe 12. The intercooler 16
cools the cathode gas to be supplied to the FC 4. The back pressure
valve 17 adjusts a back pressure on the cathode side of the FC 4.
Driving of each of the air compressor 14, the bypass valve 15, and
the back pressure valve 17 is controlled by the ECU 3.
[0039] During normal power generation of the FC4, the cathode gas
from the air compressor 14 is controlled to be in a supply state in
which the cathode gas is supplied to the FC 4 by adjusting opening
degrees of the bypass valve 15 and the back pressure valve 17. In
the supply state, the bypass valve 15 communicates at least the
upstream side and the downstream side of the supply pipe 11 from
the bypass valve 15, and the back pressure valve 17 adjusts at
least the discharge pipe 12 to be in an open state excluding a
fully closed state. In addition, as will be described in detail
later, in a state in which power generation of the FC 4 is paused,
the cathode gas is controlled to be in a bypass state of bypassing
the FC 4, by using the bypass valve 15 and the back pressure valve
17, so as not to be supplied to the FC 4 but to be discharged to
the outside. In the bypass state, while the back pressure valve 17
fully closes the discharge pipe 12, and the bypass valve 15
communicates the upstream side of the supply pipe 11 from the
bypass valve 15 and the bypass pipe 13 with each other while
blocking the upstream side and the downstream side of the supply
pipe 11 from the bypass valve 15. Thus, the bypass valve 15 is an
example of a switching mechanism that can switch the commutation
state from the supply state to the bypass state.
[0040] The anode gas supply system 20 supplies hydrogen gas as the
anode gas to the FC4, and includes: a tank 20T; a supply pipe 21; a
discharge pipe 22; a circulation pipe 23; a tank valve 24; a
pressure regulating valve 25; an injector (hereinafter, referred to
as an INJ) 26; a gas-liquid separator 27; a drain valve 28; and a
hydrogen circulation pump (hereinafter, referred to as an HP) 29.
The anode inlet manifolds of the tank 20T and the FC 4 are
connected by a supply pipe 21. The tank 20T stores hydrogen gas
that is the anode gas. The discharge pipe 22 is connected to an
anode outlet manifold of the FC 4. The circulation pipe 23
communicates the gas-liquid separator 27 and the supply pipe 21
with each other. The tank valve 24, the pressure regulating valve
25, and the INJ 26 are arranged in this order from the upstream
side of the supply pipe 21. With the tank valve 24 open, the
opening degree of the pressure regulating valve 25 is adjusted, and
the INJ 26 injects the anode gas. Thereby, the anode gas is
supplied to the FC 4. The gas-liquid separator 27 and the drain
valve 28 are disposed on the discharge pipe 22 in this order from
the upstream side. The gas-liquid separator 27 separates moisture
from the anode gas discharged from the FC 4 and stores this
moisture. The water stored in the gas-liquid separator 27 is
discharged to the outside of the fuel cell system 1 through the
discharge pipe 22 when the drain valve 28 is opened. The
circulation pipe 23 is a pipe for refluxing the anode gas to the
FC4, an upstream end of the circulation pipe 23 is connected to the
gas-liquid separator 27, and the circulation pipe 23 is provided
with an HP 29. The anode gas discharged from the FC 4 is
appropriately pressurized by the HP 29 to be guided to the supply
pipe 21. Driving of each of the tank valve 24, the pressure
regulating valve 25, the INJ 26, the drain valve 28, and the HP 29
is controlled by the ECU 3.
[0041] The power control system 30 controls discharge of the FC 4
and charge-discharge of the BAT 8. The power control system 30
includes: a fuel cell DC-DC converter (hereinafter, referred to as
an FDC) 32; a battery DC-DC converter (hereinafter, referred to as
a BDC) 34; a motor inverter (hereinafter, referred to as an MINV)
38; and an auxiliary inverter (hereinafter, referred to as an AINV)
39. The FDC 32 controls output current by the FC 4 based on a
request current value transmitted from the ECU 3, and also adjusts
direct-current power from the FC4 and outputs this to the MINV 38
and the AINV 39. The BDC 34 adjusts direct-current power from the
BAT 8 and outputs this to the MINV 38 and the AINV 39. The power
generated by FC 4 can be stored in BAT 8. The MINV 38 converts the
input direct-current power into three-phase AC power and supplies
this to the motor 50. The motor 50 drives the wheels 5 to drive the
vehicle. The motor 50 functions as a power generator that generates
electricity based on kinetic energy input from the wheels 5 when
the vehicle is decelerated or runs on a slope. The power of the FC
4 and BAT 8 can be supplied to a load device other than the motor
50 via the AINV 39. Here, the load device includes, in addition to
the motor 50, auxiliary equipment for the FC 4 and auxiliary
equipment for the vehicle. The auxiliary equipment for the FC 4
includes: the air compressor 14; the bypass valve 15; the back
pressure valve 17; the tank valve 24; the pressure regulating valve
25; the INJ 26; the drain valve 28; and the HP 29, which are all
described above. The auxiliary equipment for the vehicle includes:
air conditioning equipment, a lighting device, hazard lamps, etc.,
for example.
[0042] The cooling system 40 cools the FC 4 by circulating the
cooling water through a predetermined passage, and includes: a
supply pipe 41; a discharge pipe 42; a bypass pipe 43; a radiator
44; a bypass valve 45; and a water pump (hereinafter, referred to
as a WP) 46. The supply pipe 41 is connected to a cooling water
inlet manifold of the FC 4. The discharge pipe 42 is connected to a
cooling water outlet manifold of the FC 4. The bypass pipe 43
communicates the supply pipe 41 and the discharge pipe 42 with each
other. The bypass valve 45 is provided at a connection part between
the supply pipe 41 and the bypass pipe 43. The bypass valve 45
switches the communication state between the supply pipe 41 and the
bypass pipe 43. The radiator 44 is connected to the supply pipe 41
and the discharge pipe 42. The bypass valve 45 and the WP 46 are
disposed on the supply pipe 41 in this order from the upstream
side. The WP 46 circulates the cooling water as a coolant between
the FC 4 and the radiator 44 via the supply pipe 41 and the
discharge pipe 42. The radiator 44 cools the cooling water
discharged from the FC 4 through heat-exchange with the outside
air. Driving of each of the bypass valve 45 and the WP 46 is
controlled by the ECU 3.
[0043] The ECU 3 includes: a CPU (central processing unit); a ROM
(read only memory); and a RAM (random access memory). The
accelerator opening sensor 6; an ignition switch 7; the air
compressor 14; the bypass valves 15, 45; the back pressure valve
17; the tank valve 24; the pressure regulating valve 25; the INJ
26; the drain valve 28; the HP 29; the FDC 32; the BDC 34; and the
WP 46 are all electrically connected to the ECU 3. The ECU 3
calculates a request output to the FC 4, that is, an amount of
electric power that the FC 4 is requested to output, based on the
detection value of the accelerator opening sensor 6, the driving
states of the auxiliary equipment for the vehicle and the auxiliary
equipment for the FC4, the stored power of the BAT 8, etc. Further,
the ECU 3 calculates a target current value of the FC 4 according
to the request output to the FC4, and controls the FDC 32 while
controlling the flow rate of the cathode gas and the flow rate of
the anode gas supplied to the FC 4 by the air compressor 14 and the
INJ 26 such that the output current value of the FC 4 becomes the
target current value. The ECU 3 is an example of a control unit
that executes recovery processing described later.
[0044] Further, the ECU 3 pauses or restarts power generation of
the FC 4 according to the request output to the FC 4. The pause of
the power generation of the FC 4 is realized as the ECU 3 sets the
target current value to zero and controls the FDC 32 such that the
current value swept from the FC 4 becomes zero, or as electrical
connection between the FC 4 and the load device is blocked by a
switch provided inside the FDC 32. Further, the restart of the
power generation of the FC 4 is realized as the ECU 3 sets the
target current value to a value greater than zero and controls the
FDC 32 to set the current value swept from the FC 4 to a value
greater than zero, or as the FC 4 and the load device are
electrically connected to each other by the switch provided inside
the FDC 32. In addition, the ECU 3 performs an intermittent
operation and a regenerative operation described below.
[0045] Intermittent Operation
[0046] When the request output to the FC 4 becomes equal to or
smaller than a predetermined value, the ECU 3 performs the
intermittent operation that temporarily pauses the power generation
of the FC 4 to maintain the open circuit voltage of the FC 4 within
a target range. The case in which the request output to the FC 4 is
equal to or smaller than a predetermined value includes such a
case, for example, in which the vehicle is stopped and electric
power amount requested by the auxiliary equipment for the vehicle
is small. In the intermittent operation, specifically, the anode
gas is sufficiently supplied to the FC4, and supply of the anode
gas is then stopped so as to adjust the flow rate of the cathode
gas supplied to the FC 4 in such a manner that oxygen concentration
at the cathode electrode of the FC 4 becomes lower than that during
the normal power generation. In this state, a so-called cross leak
causes the oxygen concentration at the cathode electrode of the FC
4 to become gradually lowered with time, and open circuit voltage
of the FC 4 is also gradually lowered. The cross leak is a
phenomenon in which hydrogen permeates through an electrolyte
membrane from the anode side to the cathode electrode side, and
whereby hydrogen and oxygen react with each other on the cathode
electrode side to generate water, and thus the oxygen concentration
on the cathode electrode side is lowered. When the oxygen
concentration on the cathode electrode side is lowered, the open
circuit voltage of the FC 4 is lowered. When the open circuit
voltage is equal to or smaller than a lower limit value of the
target range, the flow rate of the cathode gas supplied to the FC 4
is increased; therefore, the oxygen concentration at the cathode
electrode of the FC 4 is increased, and thus the open circuit
voltage is also increased. When the open circuit voltage becomes
equal to or greater than an upper limit value of the target range,
the flow rate of the cathode gas supplied to the FC 4 is decreased
again, and the open circuit voltage of the FC 4 is thus
lowered.
[0047] As described above, through the repetitive switching between
increase and decrease of the flow rate of the cathode gas to the FC
4, the open circuit voltage of the FC 4 is converged within the
target range. Thus, the magnitude of the open circuit voltage of
the FC 4 is proportional to the oxygen concentration at the cathode
electrode in the FC 4. Hence, the oxygen concentration at the
cathode electrode in the FC 4 can be maintained within a
predetermined range by maintaining the open-circuit voltage within
the target range in a state in which the power generation is
paused; therefore, when the request output is increased to a
predetermined value or greater and the power generation of the FC 4
is restarted, the output of the FC 4 can be immediately
increased.
[0048] Note that the target flow rate of the cathode gas when the
open circuit voltage is equal to or greater than the upper limit
value of the target range is set to a relatively low flow rate at
which the open circuit voltage does not become increased even if
the cross leakage is taken into account. This is because the rate
of decrease in oxygen concentration due to the cross leak varies
depending on the usage environment and the usage time of the FC 4,
which makes it difficult to assume this rate of decrease in
advance. Further, the target flow rate of the cathode gas when the
open circuit voltage is equal to or smaller than the lower limit
value of the target range is set to a flow rate at which the open
circuit voltage becomes increased even if the decrease in oxygen
concentration due to the cross leak is taken into account. In the
present embodiment, the target flow rate of the cathode gas, which
is set when the open circuit voltage becomes equal to or greater
than the upper limit value of the target range, is zero; however,
the present disclosure is not limited to this, and any flow rate
may be adopted as far as the open circuit voltage is lowered at
this flow rate.
[0049] The switching of the flow rate of the cathode gas is
controlled by adjusting the opening degree of the bypass valve 15
and the opening degree of the back pressure valve 17 as described
above. For example, in order to switch the flow rate of the cathode
gas to zero, it is controlled to set the communication state to the
above-described bypass state while the air compressor 14 is
maintained to rotate at a minimum rotation speed. In order to
increase the flow rate of the cathode gas, the back pressure valve
17 opens the discharge pipe 12, and the upstream side and the
downstream side of the supply pipe 11 from the bypass valve 15 are
allowed to communicate with each other by the bypass valve 15. The
control on the flow rate of the cathode gas is not limited to this,
and for example, in a supply state in which the cathode gas can be
supplied to the FC 4, the flow rate of the cathode gas to the FC 4
may be controlled to zero by stopping the driving of the air
compressor 14. During the intermittent operation, the supply of the
anode gas is stopped in a state in which the FC 4 is sufficiently
filled with the anode gas.
[0050] Regenerative Operation
[0051] When the vehicle is decelerated or runs on a slope, the ECU
3 controls the motor 50 to operate as the power generator to obtain
braking force on the vehicle and charges the BAT 8 with at least
part of generated regenerative power. Here, when the regenerative
power is equal to or greater than a predetermined value, in a state
in which the power generation of the FC 4 is temporarily paused and
the flow rate of the cathode gas supplied to the FC 4 is limited,
the ECU 3 executes the regenerative operation that consumes at
least part of the regenerative power by driving of the air
compressor 14. Specifically, as described above, the opening degree
of the bypass valve 15 and the opening degree of the back pressure
valve 17 are controlled such that the cathode gas is discharged to
the outside without being supplied to the FC 4, and the rotational
speed of the air compressor 14 is set at a maximum rotational
speed. Accordingly, surplus electric power is consumed by the air
compressor 14, and thus it is possible to prevent overcharge of the
BAT 8 resulting from excessively great regenerative electric power
with which the BAT 8 is charged. Even during the regenerative
operation, the power generation of the FC 4 is paused and the
cathode gas is not supplied; therefore, the open circuit voltage is
gradually lowered due to the above-described cross leak.
[0052] Recovery Processing
[0053] The ECU 3 is capable of executing the recovery processing
for recovering the FC 4 from deterioration of power generation
performance of the FC 4 due to deterioration of performance of the
catalyst. The recovery processing is functionally realized by the
CPU, the ROM, and the RAM of the ECU 3. The recovery processing is
processing that lowers the voltage of the FC 4 to a target value
that is a voltage value at which the electrode catalyst of the FC 4
undergoes a reduction reaction so as to remove poisonous substances
and oxide films adhering to the catalyst of the FC 4, to thereby
recover the power generation performance of the FC 4. As will be
described later in detail, in the recovery processing, the flow
rate of the cathode gas supplied to the FC 4 is limited as well as
the voltage of the FC 4 is lowered to the target value.
IV Characteristics
[0054] The ECU 3 can change current-voltage characteristics
(hereinafter, referred to as IV characteristics) of the FC 4 by
controlling the flow rate of the cathode gas supplied to the FC 4.
Specifically, the cathode stoichiometric ratio can be changed by
changing the flow rate of the cathode gas supplied to the FC 4, and
as a result, the IV characteristics of the FC 4 can be changed. The
"stoichiometric ratio" indicates a ratio of amount of a reactive
gas to be supplied relative to theoretical amount of a reactive gas
based on the requested power generation amount. In the normal power
generation condition, the flow rate of the cathode gas and the flow
rate of the anode gas are adjusted such that both the cathode
stoichiometric ratio and the anode stoichiometric ratio are
sufficiently high, to thereby perform the power generation with
high efficiency. The flow rate of the anode gas is controlled to
maintain the anode stoichiometric ratio at a sufficiently high
level regardless of the magnitude of the cathode stoichiometric
ratio so as not to cause so-called hydrogen shortage.
[0055] FIG. 2 is a graph showing IV curves C1 to C4 of the FC 4. In
the order from the IV curve C1 to the IV curve C4, the IV curves in
a case in which only the cathode stoichiometric ratio is lowered
while the anode stoichiometric ratio is sufficiently high; and the
power generation efficiency is lowered in this order. The IV curve
C1 indicates an IV curve in a state of having a sufficiently high
cathode stoichiometric ratio, and exhibits IV characteristic of the
FC 4 in the normal power generation condition. In addition, FIG. 2
shows operating points D1 to D4 and E1 to E4, a theoretical
electromotive voltage E0, a target value .alpha., a threshold value
.beta., an idle current value A1, and a current value A4. Note that
the shift of the operating points of the FC 4 shown in the graph is
realized by control by the ECU 3 on the flow rate of the anode gas
supplied to the FC 4 within a range where no hydrogen shortage
occurs, while control by the ECU 3 on the flow rate of the cathode
gas supplied to the FC 4, and also control on the FDC 32 such that
a sweep current value from the FC 4 reaches the target current
value.
[0056] The operating points D1 to D4 are respective operating
points on the IV curve C1 to the IV curve C4. The theoretical
electromotive voltage E0 of the FC 4 is obtained by multiplying the
theoretical electromotive voltage of a single cell by the number of
stacked single cells of the FC 4. The operating points E1 to E4 are
operating points at which the current becomes zero on the
respective IV curves C1 to C4. The target value .alpha. is a target
value for the above-described recovery processing. The threshold
value .beta. is an open circuit voltage value at the operating
point E4, and is a higher value than the target value .alpha.. The
threshold value .beta. is a value smaller than the open-circuit
voltage value at the operating point E1 on the IV curve C1
indicating the IV characteristics in the normal power generation
condition. For example, the threshold value .beta. is set to a
value obtained by multiplying 0.6 V by the number of stacked single
cells of the FC 4. The idle current value A1 is an output current
value while the FC 4 is in an idle operation state. The current
value A4 is a current value when the voltage reaches the target
value .alpha. on the IV curve C4. The current value A4 is a value
smaller than the idle current value A1.
[0057] The idle operation state is a state, for example, in which
the vehicle is temporarily stopped, and the FC 4 generates only
power equivalent to power consumed by the auxiliary equipment for
the FC 4, which is minimum necessary to continue the power
generation of the FC 4. The operating point D1 indicates an
operating point on the IV curve C1 in the idling operation state.
The operating point D4 indicates an operating point on the IV curve
C4 when the voltage reaches the target value .alpha.. The operating
points D2 and D3 will be described later.
[0058] As shown in FIG. 2, the output voltage is lowered as the
output current of the FC 4 is increased. Here, the power generation
amount of the FC 4 can be obtained by multiplying the output
current value of the FC 4 by the output voltage value; and in
general, the power generation amount is increased as the output
current is increased. For example, when the power generation
efficiency of the FC 4 is high as indicated by the IV curve C1, in
order to lower the output voltage of the FC 4 to the target value
.alpha. in the recovery processing, it is necessary to greatly
increase the output current of the FC 4, that is, the output power
of the FC 4 is required to be greatly increased. However, for
example, as shown by C4, when the power generation efficiency of
the FC 4 is low, in order to lower the output voltage of the FC 4
to the target value .alpha., only small output current is required
for the FC 4, that is, it is unnecessary to greatly increase the
output power of the FC 4. Therefore, for example, when the request
output to the FC 4 is small, by executing the recovery processing
while lowering the power generation efficiency, it is possible to
reduce increase in power generation amount of the FC 4 due to the
execution of the recovery processing, to thereby prevent increase
in surplus power as well as prevent the FC 4 from having a higher
temperature. In the comparative example described below, while the
power generation efficiency of the FC 4 is lowered, the recovery
processing is executed by shifting the operating point in the order
from the operating points D1, D2, D3 to D4.
[0059] Recovery Processing in Comparative Example
[0060] FIG. 3A, FIG. 3B, FIG. 3C, and FIG. 3D are explanatory
diagrams of the recovery processing in the comparative example. In
the recovery processing of the comparative example, from the
operating point D1 in the idle operation state, the flow rate of
the cathode gas is decreased while the power generation of the FC 4
is continued, to thereby lower the cathode stoichiometric ratio,
and thus the operating point is shifted in the order from D2, D3 to
D4. The sweep current value is gradually decreased from the idle
current value A1 to the current value A4. Each of FIG. 3A to FIG.
3D shows a power generation amount H1 and a heat generation amount
H2 of the FC 4 at the operating points D1 to D4. The power
generation amount H1 at the operating point D1 can be obtained by
multiplying the idle current value A1 and the voltage value at the
operating point D1. The heat generation amount H2 at the operating
point D1 can be obtained by multiplying a value, which is obtained
by subtracting the voltage value at the operating point D1 from the
theoretical electromotive voltage E0, by the current value at the
operating point D1. Similarly, the power generation amount H1 at
the operating point D2 can be obtained by multiplying the current
value and the voltage value at the operating point D2. The heat
generation amount H2 at the operating point D2 can be obtained by
multiplying a value, which is obtained by subtracting the voltage
value at the operating point D2 from the theoretical electromotive
voltage E0, by the current value at the operating point D2. The
power generation amount H1 at the operating point D3 can be
obtained by multiplying the current value and the voltage value at
the operating point D3. The heat generation amount H2 at the
operating point D3 can be obtained by multiplying a value, which is
obtained by subtracting the voltage value at the operating point D3
from the theoretical electromotive voltage E0, by the current value
at the operating point D3. The power generation amount H1 at the
operating point D4 can be obtained by multiplying the current value
and the voltage value at the operating point D4. The heat
generation amount H2 at the operating point D4 can be obtained by
multiplying a value, which is obtained by subtracting the voltage
value at the operating point D4 from the theoretical electromotive
voltage E0, by the current value at the operating point D4.
[0061] In the recovery processing of the comparative example, since
the FC 4 constantly generates power in the process of shifting from
the operating point D1 to the operating point D4, the power
generation amount H1 and the heat generation amount H2 are always
generated. Hence, in the recovery processing of the comparative
example, it is desired to further reduce surplus power and suppress
the FC 4 from having a higher temperature.
[0062] Recovery Processing of Present Embodiment
[0063] FIG. 4A and FIG. 4B are explanatory diagrams of the recovery
processing of the present embodiment. In the present embodiment,
from the operating point E1 at which the power generation of the FC
4 is paused, the flow rate of the cathode gas supplied to the FC 4
is controlled so as to lower the IV characteristics, and then the
power generation is restarted at the operating point E4 and the
operating point is shifted to D4. The shift of the operating point
from E1 to E4 can be realized by the above-described cross leak
caused by decreasing the flow rate of the cathode gas while the
power generation of the FC 4 is paused. Here, while the power
generation of the FC 4 is paused, the power generation amount H1
and the heat generation amount H2 are naturally not generated. As
shown in FIG. 4B, when the open circuit voltage of the FC 4 becomes
equal to or smaller than the threshold value .beta. at the
operating point E4, the power generation of the FC 4 is restarted
and the operating point is shifted from E4 to D4. In the recovery
processing of the present embodiment, the power generation amount
H1 and the heat generation amount H2 are generated at the operating
point D4. Also in the present embodiment, the power generated by
the execution of the recovery processing is stored in the BAT
8.
[0064] Here, the output current value A4 at the operating point D4
is smaller than the idle current value A1. That is, the output
current value A4 of the FC 4, which corresponds to the target value
.alpha. and is determined by the IV curve C4 corresponding to the
threshold value .beta., is smaller than the idle current value A1.
The threshold value .beta. is previously determined based on
experimental results such that the output current value is not
greater than the idle current value A1 before the output voltage
value reaches the target value .alpha., as long as the flow rate of
the cathode gas is not increased when the FC 4 power generation is
restarted at the time point when the open circuit voltage reaches
the threshold value .beta..
[0065] As described above, in the present embodiment, since the
recovery processing is executed after the open circuit voltage is
lowered to the threshold value .beta. or less in a state in which
the power generation of the FC 4 is paused, the power generation
amount H1 and the heat generation amount H2 are more greatly
decreased as compared with the case where the recovery processing
is executed while the power generation is continued, as in the
comparative example. Since the power generation amount H1 is small,
it is possible to reduce surplus power caused by the execution of
the recovery processing, and thus the BAT 8 can be suppressed from
being overcharged when the BAT 8 is charged with the surplus power.
Furthermore, since the heat generation amount H2 is also small, the
FC 4 can be suppressed from having a higher temperature. In
particular, while the vehicle is stopped, traveling wind does not
hit the radiator 44, so that it is difficult to sufficiently lower
the temperature of the cooling water of the FC 4; however, even in
this case, it is possible to suppress the FC 4 from having a higher
temperature.
[0066] Control Executed by ECU 3
[0067] Next, the control executed by the ECU 3 will be specifically
described. FIG. 5 is a flowchart showing an example of the control
of the present embodiment. This control is repetitively executed.
First, the ECU 3 determines whether or not there is an execution
request of the recovery processing (hereinafter, referred to as a
recovery request) (step S1). For example, it is determined that
there is a recovery request if at least one of the following
conditions is satisfied: when the voltage value at a predetermined
current density of the FC 4 is smaller than the threshold value;
when the elapsed time from the execution of the previous recovery
processing is a predetermined time or longer; when the operation
time of the FC 4 from the execution of the previous recovery
processing is a predetermined time or longer, or when the vehicle
travel distance from the execution of the previous recovery
processing is a predetermined distance or more. In the case of No
in step S1, this control is terminated.
[0068] In the case of Yes in step S1, the ECU 3 determines whether
or not the intermittent operation is in progress (step S3). In the
case of Yes in step S3, the ECU 3 determines whether or not the
regenerative operation is in progress (step S5). In the case of No
in step S3, the recovery processing is not executed and this
control is thus terminated. In the case of No in step S5, the
recovery processing is not executed and this control is thus
terminated.
[0069] In the case of Yes in step S3 and in step S5, the ECU 3
determines whether or not the open circuit voltage of the FC 4 is
equal to or smaller the threshold value .beta. (step S7). The
threshold value .beta. is acquired in advance through an experiment
and stored in the ROM of the ECU 3. In the case of No in step S7,
this control is terminated. In the case of Yes in step S7, the ECU
3 executes the recovery processing (step S9). Specifically, the
power generation of the FC 4 is restarted by the above-described
method, and the sweep current value of the FC 4 is controlled to be
the current value A4. As the voltage of the FC 4 becomes the target
value .alpha. or smaller, the power generation performance of the
FC 4 is restored. Note that after the completion of the recovery
processing, the ECU 3 controls the power generation state of the FC
4 according to the requested operation state.
[0070] Both step S3 and step S5 are processing for determining
whether or not the power generation of the FC 4 is paused.
Therefore, in the case in which the power generation of the FC 4
can be controlled to be paused other than in the intermittent
operation or the regenerative operation, it may be determined
whether or not the power generation of the FC 4 is in a pause
state, instead of step S3 and step S5.
[0071] Recovery Processing Executed During Intermittent
Operation
[0072] FIG. 6 is a timing chart when the recovery processing is
executed during the intermittent operation. FIG. 6 shows the
request output to the FC 4, the execution state of the intermittent
operation, the flow rate of the cathode gas supplied to the FC 4,
the voltage and the current of the FC 4, and the execution state of
the recovery processing. Further, FIG. 6 shows an upper limit value
V1 and a lower limit value V2 of the target range of the open
circuit voltage by the intermittent operation.
[0073] When the request output to the FC 4 starts to become
decreased while the FC 4 is in the power generation state, the ECU
3 controls to gradually decrease the flow rate of the cathode gas
supplied to the FC 4; thus, the output current of the FC 4 is
decreased and the output voltage thereof is increased such that the
output power of the FC 4 is decreased. When the request output to
the FC 4 becomes a predetermined value .beta. or smaller, the
normal operation is switched to the intermittent operation (time
t1).
[0074] Here, the above-described threshold value .beta. is set to
be a value smaller than the upper limit value V1 and greater than
the lower limit value V2. Therefore, in the case of having a
recovery request, before the open circuit voltage is lowered to the
lower limit value V2, the open circuit voltage becomes equal to or
smaller than the threshold value .beta., and thus the intermittent
operation is temporarily paused, and the recovery processing is
executed (time t2). When the recovery processing is executed, the
power generation of the FC 4 is restarted while the flow rate of
the cathode gas under control during the intermittent operation is
limited to zero, so that the sweep current value of the FC 4 is
controlled to be the current value A4, and whereby the output
voltage is lowered to the target value .alpha. or smaller. When the
output voltage reaches the target value .alpha., the swept current
value is controlled to be zero again, and the recovery processing
is then completed (time t3). When the request output becomes
greater than the predetermined value P after the completion of the
recovery processing, the flow rate of the cathode gas is increased,
to thus restart the power generation of the FC 4.
[0075] The example of FIG. 6 shows the case in which the open
circuit voltage is not increased even when the current value is
controlled to be zero after the completion of the recovery
processing. This is because the oxygen concentration in the FC 4 is
lowered because the oxygen in the FC 4 is consumed because of the
execution of the recovery processing, that is, the restart of the
power generation, so that the oxygen concentration is lower after
the completion of the recovery processing than immediately before
the start of the recovery processing, and thus the open circuit
voltage does not become higher. Further, during the intermittent
operation having no recovery request, the open circuit voltage is
maintained between the upper limit value V1 and the lower limit
value V2 regardless of the magnitude of relationship between the
open circuit voltage and the threshold value .beta..
[0076] Recovery Processing Executed During Regenerative
Operation
[0077] FIG. 7 is a timing chart when the recovery processing is
executed during the regenerative operation. FIG. 7 shows the
vehicle speed, the regenerative power generation amount, the
execution state of the regenerative operation, the flow rate of the
cathode gas supplied to the FC 4, the voltage and the current of
the FC 4, and the execution state of the recovery processing.
[0078] For example, the vehicle speed is accelerated during running
on a slope and the regenerative power generation amount of the
motor 50 starts to be increased; although not shown in the drawing,
the request output to the FC 4 is decreased, and the flow rate of
the cathode gas supplied to the FC 4 is gradually decreased; and
the output current of the FC 4 is decreased and the output voltage
thereof is increased. When the regenerative power generation amount
becomes greater than a threshold value G that is smaller by a
predetermined margin than a charge limit value L at which the BAT 8
can be charged, the normal operation is switched to the
regenerative operation (time t11). After switching to the
regenerative operation, as described above, the cathode gas is
switched to a state of not being supplied to the FC 4 by the bypass
valve 15 and the back pressure valve 17, and the air compressor 14
is driven by the regenerative electric power of the motor 50.
Thereby, the regenerative power can be consumed, so that the BAT 8
is suppressed from being charged with the regenerative power
greater than the charge limit value L. Further, the cathode gas
from the air compressor 14 is discharged to the outside without
being supplied to the FC 4. Hence, the open circuit voltage is
gradually lowered to be equal to or smaller than the threshold
value .beta., and then the recovery processing is executed (time
t12). Thereafter, when the open circuit voltage becomes equal to or
smaller than the target value .alpha., the recovery processing is
completed (time t13). In this manner, the recovery processing is
executed while the regenerative operation is continued.
[0079] Recovery Processing of First Modification
[0080] Next, a plurality of modifications of the recovery
processing will be described. FIG. 8A and FIG. 8B are explanatory
diagrams of the recovery processing of the first modification. FIG.
8A and FIG. 8B show a IV curve C5, operating points E5 and D5, a
threshold value .beta.a, and an output current value A5. The IV
curve C5 is an IV curve whose power generation efficiency is
further lowered than that of the IV curve C4. The operating point
E5 indicates an operating point on the IV curve C5 when the output
current value is zero. The operating point D5 indicates an
operating point on the IV curve C5 when the output voltage becomes
the target value .alpha.. The output current value A5 is an output
current value defined based on the IV curve C5 and when the output
voltage is equal to the target value .alpha.. The threshold value
Pa is an open circuit voltage value at the operating point E5, and
is a value smaller than the above-described threshold value .beta..
In the first modification, after the operating point is shifted
from E1 to E5 and the open circuit voltage is lowered to or lower
than the threshold value .beta.a, the power generation is restarted
and the operating point is shifted to D5, and the recovery
processing is executed. Accordingly, the power generation amount H1
and the heat generation amount H2 can be decreased more in the
first modification than in the present embodiment.
[0081] FIG. 9 shows a timing chart when the recovery processing of
the first modification is executed during intermittent operation.
The threshold value .beta.a is set to be smaller than the lower
limit value V2 of the target range of the open circuit voltage
during the intermittent operation. When there is a recovery
request, the flow rate of the cathode gas is not increased even
when the open circuit voltage becomes the lower limit value V2 or
smaller, and the recovery processing is executed when the open
circuit voltage becomes the threshold value .beta.a or smaller
(time t2a). That is, after the oxygen concentration at the cathode
electrode is sufficiently lowered, the power generation of the FC 4
is restarted and the recovery processing is executed. Therefore,
the output voltage of the FC 4 drops to the target value .alpha. or
smaller in a short period after the power generation is restarted,
and the recovery processing is completed in a short period (time
t3a).
[0082] Further, since the threshold value .beta.a is equal to or
smaller than the lower limit value V2, the lower limit value V2 may
be set to be a higher value. By setting the lower limit value V2 to
be a higher value, the open circuit voltage can be maintained at a
relatively high value in the case of having no recovery request.
Accordingly, it is possible to ensure responsiveness of the output
of the FC 4 when the normal power generation is restarted.
[0083] In the first modification, the recovery processing is
executed after the open circuit voltage is sufficiently lowered
from the operating point E1 to the operating point E5. On the other
hand, in the above-described present embodiment, as shown in FIG.
3A to FIG. 3D, the recovery processing is executed after the open
circuit voltage is lowered from the operating point E1 to the
operating point E4. Accordingly, although the power generation
amount H1 and the heat generation amount H2 can be decreased more
in the first modification than in the present embodiment, it takes
more time until the power generation of the FC 4 is restarted,
which means that it takes more time until the recovery processing
is executed. Hence, considering and comparing the effect of
decreasing the power generation amount H1 and the heat generation
amount H2 as well as the period until the recovery processing is
executed, it is preferable to appropriately determine a threshold
value used for executing the recovery processing. Note that in FIG.
9, because of the space constraints, it is illustrated that the
drop rate of the open circuit voltage due to the intermittent
operation is greater than that in FIG. 6, but in practice, both are
substantially the same.
[0084] Recovery Processing of Second Modification
[0085] Next, the recovery processing of the second modification
will be described. FIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D are
explanatory diagrams of the recovery processing of the second
modification. In the second modification, the threshold value
.beta.b is set to be a value greater than the above-described
threshold value .beta., and is set to be the same value as the
value of the open circuit voltage at the operating point E2. In the
second modification, the operating point is shifted in the order
from E1, E2, D2, D3, to D4. The shift of the operating point from
E2 to D2 can be realized by restarting the power generation of the
FC 4 and increasing the sweep current to the FC 4 within a range
less than the idle current value A1. The shift of the operating
point from D2 to D4 can be realized by gradually decreasing the
sweep current to the FC 4. Also in the second modification, since
the IV characteristic is lowered while the power generation of the
FC 4 is paused, the power generation amount H1 and the heat
generation amount H2 can be more greatly decreased than in the
comparative example shown in FIG. 3A to FIG. 3D.
[0086] FIG. 11 is a timing chart when the recovery processing of
the second modification is executed during the intermittent
operation. The threshold value .beta.b is the same as the
above-described threshold value .beta. in that this threshold value
.beta.b is smaller than the upper limit value V1 of the
intermittent operation and greater than the lower limit value V2 of
the intermittent operation, but the threshold value .beta.b is a
value closer to the upper limit value V1. Hence, the power
generation of the FC 4 is restarted in a short period after the
intermittent operation is executed and the recovery processing is
then executed (time t2b), and whereby the output voltage can be
lowered to the target value .alpha. (time t3b).
[0087] When the present embodiment shown in FIG. 4A and FIG. 4B is
compared with the second modification, the power generation amount
H1 and the heat generation amount H2 can be more greatly decreased
in the present embodiment. However, when the flow rate of the
cathode gas is limited under the same conditions, in the second
modification, the time required for the shift of the operating
point from E1 to D4 is shorter than the time required for the shift
of the operating point from E1 to D4 in the recovery processing of
the present embodiment. For example, the period from the time t1 to
the time t3b shown in FIG. 11 is shorter than the period from the
time t1 to the time t3 shown in FIG. 6. Both the present embodiment
and the second modification have the same period for the shift of
the operating point from E1 to E2, and the period for the shift of
the operating point from E4 to D4 in the present embodiment can be
considered to be substantially the same as the period for the shift
of the operating point from E2 to D2 in the second modification.
However, the drop of the voltage from the operating point D2 to the
operating point D4 in the second modification is realized by
consumption of oxygen in the cathode gas by the power generation of
the FC 4. On the other hand, the drop of the open circuit voltage
from the operating point E2 to the operating point E4 in the
present embodiment is realized by consumption of oxygen due to the
cross leak. Here, in general, the consumption rate of oxygen by the
power generation is faster than the consumption rate of oxygen by
the cross leak. Therefore, the effect of decreasing the power
generation amount H1 and the heat generation amount H2 is smaller
in the second modification than in the present embodiment, but the
time until the recovery processing is completed can be more greatly
shortened than that in the present embodiment. Accordingly,
considering and comparing the effect of decreasing the power
generation amount H1 to the heat generation amount H2 as well as
the period until the recovery processing is completed, it is
preferable to appropriately set a threshold value for executing the
recovery processing.
[0088] Switching of Recovery Processing of Present Embodiment,
First Modification, and Second Modification
[0089] Depending on the situation, the recovery processing executed
in each of the present embodiment, the first modification, and the
second modification may be switched as appropriate. For example, as
described above, the electric power of the FC 4 generated by the
execution of the recovery processing is stored in the BAT 8; and
when the remaining charge in the BAT 8 is smaller than a
predetermined value, the recovery processing of the present
embodiment may be executed, but when the remaining charge is equal
to or greater than the predetermined value, the recovery processing
of the first modification may be executed. In this manner, when the
remaining charge in BAT 8 is relatively great, by executing the
recovery processing of the first modification, amount of the power
generation generated by the recovery processing may be further
decreased so as to prevent overcharging of the BAT 8.
[0090] In addition, there may occur a case in which the request
output to the FC 4 is increased to the predetermined value P or
greater before the open circuit voltage becomes equal to or smaller
than the threshold value .beta. in a state of having a recovery
request, so that the normal power generation is restarted but the
recovery processing is not executed. If such a situation occurs
repeatedly, the recovery processing of the second modification
using the threshold value .beta.b, which is greater than the
threshold value (3, may be executed. This configuration ensures an
opportunity for executing the recovery processing. Specifically,
the number of times that the intermittent operation and the
regenerative operation are executed without executing the recovery
processing in a state of having a recovery request may be counted,
and the threshold value .beta. may be used when the count value is
smaller than a predetermined value, and the threshold value .beta.b
may be used when the count value is equal to or greater than the
predetermined value. In addition, the time during which the
recovery processing is not executed in a state of having a recovery
request is measured, and the threshold value .beta. may be used
when the measured time is shorter than a predetermined time, and
the threshold value .beta.b may be used when the measured time is
equal to or longer than the predetermined time.
[0091] Further, the threshold value .beta.a may be used during the
regenerative operation, and the threshold value .beta. or .beta.b
may be used during the intermittent operation. During the
regenerative operation, since reduction in surplus power due to the
power generation by the FC 4 is desirable, and reduction in surplus
power can be attained by executing the recovery processing of the
present embodiment with a smaller power generation amount. In
addition, there may be a case in which during the intermittent
operation, depending on the driving operation by a driver, the
request output to the FC 4 is increased to the predetermined value
P or greater before the open circuit voltage becomes equal to the
threshold value .beta.a or smaller, and thus the normal power
generation is restarted, which may make it impossible to ensure an
opportunity for executing the recovery processing.
[0092] Other Remarks
[0093] In the present embodiment, the recovery processing can be
executed in either of the intermittent operation and the
regenerative operation, but the recovery processing may be executed
only in any one of these operations.
[0094] Although the preferred embodiments of the present disclosure
have been described in detail above, the present disclosure is not
limited to such specific embodiments, and various modifications and
changes can be made within the scope of the gist of the present
disclosure as set forth in the claims.
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